1. Field of the Invention
The present invention relates to automatic warehouses including (i) a rack including vertically arranged shelves, each of the shelves including trays horizontally arranged, and (ii) traveling vehicles each of which is provided to a corresponding one of the shelves, and independently travels in a horizontal direction.
2. Description of the Related Art
Some conventional automatic warehouses include a rack having shelves vertically arranged, and traveling vehicles to convey articles from and to the rack. Here each of the shelves has a row of trays horizontally arranged.
The operation of such automatic warehouses requires efficiency in time and space. A technique has disclosed traveling vehicles used for efficient storage and retrieval of articles (See, for example, Japanese Unexamined Patent Application Publication No. 11-278607).
The technique disclosed in Japanese Unexamined Patent Application Publication No. 11-278607 shows that each of the traveling vehicles horizontally travels along one of the shelves, and has an elevating platform equipped with a transfer device. In addition to loading and unloading of articles to and from the shelf, the traveling vehicle can load and unload the articles to and from a shelf located one level or two levels lower than the original shelf.
Here there is an automatic warehouse built for an effective use of storage space and efficient storage and retrieval operations in the automatic warehouse. The automatic warehouse has stations each provided to a different position at one rack. Each station independently stores and retrieves the articles.
In such an automatic warehouse, for example, each station handles a different amount of articles (storage and retrieval amount) to be placed on and taken out of the racks. Thus, the point is how the storage space in the automatic warehouse is assigned to each station. Specifically, it is important to figure out how trays of the rack should be assigned.
FIG. 12 exemplifies a technique to assign trays of a rack to two stations in a conventional automatic warehouse.
A conventional automatic warehouse 500 shown in FIG. 12 includes the following: a rack 500 having shelves 555 vertically arranged, two stations (a first station 512 and a second station 522) for storage and retrieval, and two stacker cranes (a first stacker crane 510 and a second stacker crane 520).
Specifically, in FIG. 12, the far left and the far right of the rack 550 respectively represent the home position and the opposite home position. Here the first station 512 and the second station 522 are respectively provided at the home position and the opposite home position.
The first stacker crane 510 can transport an article, which is brought to the first station 512 from outside, to one of the trays on the rack 550. The first stacker crane 510 can also unload an article from one of the trays on the rack 550, and transport the article to the first station 512.
Similarly, the second stacker crane 520 can (i) transport an article from the second station 522 to the rack 550, and (ii) unload an article from the rack 550 and transport the article to the second station 522.
The first stacker crane 510 and the second stacker crane 520 travel on the same lane horizontally running in front of the rack 550. Thus, the first stacker crane 510 and the second stacker crane 520 cannot overtake each other.
Under such restrictions, assume for example the case where the storage and retrieval amount that the second station 522 handles is greater than that the first station 512 does. Here, FIG. 12 shows the following: a group of tray columns 570, including tray columns of the rack 550 from the opposite home position to the fifth column, is assigned as tray columns used for the second station 522, and a group of tray column 560, including the rest of the three tray columns, is assigned as tray columns used for the first station 512.
Regarding the two stations for the storage and retrieval of the articles, the technique involves determining how the tray columns are assigned to each station so that one of the two stations, handling a greater storage and retrieval amount, has more trays assigned to it.
It is noted that the assigning is carried out by, for example, a controller controlling operations of the automatic warehouse 500.
Thanks to such assigning, the automatic warehouse 500 can make sure to store and retrieve the articles to be stored and retrieved via each of the first station 512 and the second station 522. Furthermore, the first stacker crane 510 and the second stacker crane 520 can technically store and retrieve the articles, freeing each from interference from the other.
Unfortunately, the conventional technique has a problem. That is, as one of the two storage and retrieval stations handles more storage and retrieval amount than the other, the stacker crane for the station handling more storage and retrieval amount has to travel a greater distance.
In other words, one of the stacker cranes has to travel a greater distance as the difference becomes greater in storage and retrieval amount between the two storage and retrieval stations. Thus, the station which has to handle more storage and retrieval amount suffers a decrease in storage and retrieval amount of articles per unit time. The resulting problem is a decrease in capability of the overall automatic warehouse for storage and retrieval.